Method for producing graphite nanocatalysts having improved catalytic properties

ABSTRACT

High temperature treatment of graphite nanofibers to increase their catalytic activity. The heat treated graphite nanofiber catalysts are suitable for catalyzing chemical reactions such as oxidation, hydrogenation, oxidative-dehydrogenation, and dehydrogenation.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation-in-Part of U.S. Ser. No. 10/712,247 filed Nov.13, 2003 which is based on Provisional Application U.S. Ser. No.60/426,198 filed Nov. 14, 2002.

FIELD OF THE INVENTION

This invention relates to the use of high temperature treatment ofgraphite nanofibers to increase their catalytic activity. The heattreated graphite nanofiber catalysts are suitable for catalyzingchemical reactions such as oxidation, hydrogenation,oxidative-dehydrogenation, and dehydrogenation.

BACKGROUND OF THE INVENTION

Much work has been done over the years in the field of heterogeneouscatalysis. Such catalysts have experienced enormous commercial successin many chemical processes, particularly petroleum and petrochemicalprocess applications. Conventional heterogeneous catalysts are typicallycomprised of one or more catalytically active metals, particularly GroupVIII and Group VI metals on an inorganic support. The inorganic supportis typically a metal oxide such as alumina, silica, alumina-silica,titania, magnesia, as well as molecular sieves. Various forms of carbonhave also been suggested as being suitable as catalyst supportmaterials. For example, U.S. Pat. Nos. 5,538,929 and 6,277,780 teach theuse of a phosphorus treated activated carbon as catalyst supports. Also,U.S. Pat. No. 5,972,525 teaches solid particles comprised of carbon andmetal oxides as being suitable catalyst supports. While most of the artteaches the use of conventional carbon, such as activated carbon ascatalyst supports, two patents, U.S. Pat. Nos. 5,569,635 and 6,159,892disclose the use of nano-size cylindrical carbon “fibrils” as catalystsupports. Various catalytically active metals, preferably noble andnon-noble Group VIII metals, such as Fe and Pt, are deposited onto thefibril support material. Metal oxides, such as Fe₂O₃ can also act as acatalyst when deposited onto the carbon fibrils.

While it has been known for many years that both macro and nano-sizecarbon particles are suitable support materials for certain types ofcatalysts, it has not been known that certain types of graphiticnanofibers have unique and unexpected catalytic properties themselves,without the addition of a catalytically active metal. In co-pendingapplication, U.S. Ser. No. 10/712,247, it is disclosed that graphiticnanofibers comprised of a plurality of graphite sheets aligned indirections parallel, perpendicular, or at an angle to the longitudinalaxis of the nanofiber are suitable for catalyzing a variety of chemicalreactions. It has unexpectedly been found by the inventors hereof thatif the graphite nanofibers in which the graphite sheets are orientedperpendicular, or at an angle, to the longitudinal axis are initiallytreated at high temperatures then their subsequent catalytic performanceis unexpectedly enhanced over that of the corresponding untreatedmaterials.

SUMMARY OF THE INVENTION

A catalytic process selected from oxidation, hydrogenation,dehydrogenation, oxidative-hydrogenation, and oxidative-dehydrogenationwhich is catalyzed by a catalyst composition comprised of graphiticnanofibers which nanofibers are comprised of a plurality of graphiteplatelets aligned perpendicular, or at an angle to the longitudinal axisof the nanostructure and wherein at least about 50% of the edge sites ofsaid nanofibers are exposed, wherein said graphite nanofibers, prior touse in said catalytic process are heat treated in the presence of aninert gas at temperatures from about 2300° C. to about 3000° C.

In another preferred embodiment these high temperature treated graphitenanofibers can be used as support media for metal particles.

DETAILED DESCRIPTION OF THE INVENTION

The catalysts of the present invention are graphite nanofibers. Thesegraphite nanofibers are themselves comprised of a plurality of graphiteplatelets, also sometimes called graphite sheets, that are aligned,perpendicular, or at an angle to the longitudinal (growth) axis of thenanofiber. By “at an angle” we mean that the graphite platelets arealigned so that they are neither parallel nor perpendicular to thelongitudinal axis of the nanofiber. For example they can be from about1° to about 89°, preferably from about 10° to about 80°, more preferablyfrom about 20° to about 70°, and most preferably from about 30° to about60° with respect to the longitudinal axis of the nanofiber. In the casewhere the graphitic sheets are oriented substantially perpendicular tothe growth axis, the carbon nanofibers are sometimes referred to as“platelet” nanofibers. In the case where the graphitic sheets areoriented at an angle to the growth axis, the nanofibers are sometimesreferred to as “herringbone” nanofibers. The term “carbon” is sometimesused interchangeably with “graphite” herein and the word “nanostructure”is sometimes used interchangeably with “nanofiber” herein.

The graphite nanofibers of the present invention are novel materialshaving a unique set of properties that include: (i) a surface area fromabout 20 to 50 m²/g, preferably from about 30 to 45 m²/g, more and mostpreferably from about 35 to 40 m²/g, which surface area is determined byN₂ adsorption at −196° C.; (ii) a crystallinity from about 5% to about100%, preferably from about 50% to 100%, more preferably from about 75%to 100%, most preferably from about 90% to 100%, and ideallysubstantially 100%; (iii) an average pore size from about 10 to 15 nm,most preferably from about 11 to 13 run and ideally 12 nm; (iv)interstices of about 0.335 nm to about 0.40 nm, preferably about 0.335nm; and (v) unexpectedly high catalytic properties for certain chemicalreactions. The interstices are the distance between the graphiteplatelets. The over all shape of the nanofibers can be any suitableshape. Non-limiting examples of preferred shapes include straight,branched, twisted, spiral, helical, and coiled.

The graphite nanofiber catalysts of the present invention can becatalytically grown from suitable unsupported metal powders in a carboncontaining atmosphere. A carbon-containing compound is decomposed in thepresence of the metal catalyst at temperatures from about 450° C. toabout 800° C., preferably from about 550° C. to about 700° C. It is alsopreferred that hydrogen be present during the decomposition of thecarbon-containing compound. The graphite nanofibers of the presentinvention are treated in an inert gas environment to a temperature fromabout 1800° C. to about 3000° C., preferably from about 2300° C. toabout 3000° C. Preferred inert gases are helium and argon with heliumbeing more preferred. This high temperature heat treatment is what givesthe graphite nanofibers of the present invention their unexpectedimproved catalytic properties when compared to similar graphitenanofibers that were not subjected to high temperature heat treatment.

Metal powdered catalysts suitable for growing the carbon nanofibers ofthe present invention include single metals, as well as alloys andmulti-metallics. If the metal catalyst is a single metal then it ispreferably a Group VIII metal selected from Fe, Ni, and Co. If thecatalyst is an alloy or multi-metallic material, then it is preferredthat it be comprised of a first metal component that will be one or moreGroup VIII metals and a second metal that is preferably one or moreGroup IB metals, such as Cu, Ag, and Au. Preferred are Cu and Ag with Cubeing the most preferred. It will be understood that Zn can be used inplace of one or more of the Group VIII metals. The Group IB metals arepresent in an amount ranging from about 0.5 to 99 at. % (atomic %). Forexample the catalyst can contain up to about 99 at. %, even up to about70 at. %, or even up to about 50 at. %, preferably up to about 30 at. %,more preferably up to about 10 at. %, and most preferably up to about 5wt. % of Group IB metal with the remainder being a Group VIII metal,preferably nickel or iron, more preferably iron.

Catalysts having a high copper content (70 at. % to 99 at. %) willtypically generate nanofibers that are predominantly helical or coiled,in overall shape, and which have a relatively low crystallinity (fromabout 5 to 25%). Lower concentrations of copper, e.g., 0.5 to 30 at. %have a tendency to produce spiral and branched nanofibers, whereas acatalyst with about 30 to 70 at. %, preferably 30 to 50 at. % copperwill produce predominantly branched nanofibers. A third metal can alsobe present. There is no limitation with respect to what the particularthird metal can be as long as it is not deleterious to the desired endproduct nanofiber. It is preferred that the third metal, if used, beselected from the group consisting of Ti, W, Sn and Ta. When a thirdmetal is present, it is substituted for up to about 20 at. %, preferablyup to about 10 at. %, and more preferably up to about 5 at. %, of thesecond metal. It is preferred that the catalyst be comprised of Cu incombination with Fe, Ni, or Co. More preferred is Cu in combination withFe and/or Ni from an economic point of view. A catalyst of which Fe isused in place of some of the Ni would be less expensive than a catalystcomprised of Cu in combination with only Ni. Preferred catalysts forproducing graphite nanofibers wherein the platelets are substantiallyperpendicular to the longitudinal axis of the nanofiber are Fe and Fe/Cumulti-metallics. Preferred catalysts for producing graphite nanofiberswherein the graphite platelets are at an angle, other than 90 degrees,from the growth axis, are Fe, Fe/Cu multi-metallics, Fe/Nimulti-metallics, and Ni/Cu multi-metallics. The preferred temperaturerange for growing “platelet” graphite nanofibers is from about 550° toabout 650° C., preferably from about 575° to about 625° C. The preferredtemperature range for growing the angled “herringbone” graphitenanofibers is from about 550° to about 580° C.

Any suitable method can be used to produce the powdered metal catalystfor growing the graphite nanocatalysts of the present invention. Aspreviously mentioned, it is most preferred in the practice of thepresent invention that the graphite nanocatalysts be grown fromunsupported metallic powders. A preferred method for preparing suitableunsupported metal catalytic powders is the use of colloidal techniquesfor precipitating them as metal oxides, hydroxides, carbonates,carboxylates, nitrates, etc. Such a process typically involvesdissolving salts of each metal of the catalyst in an appropriatesolvent, preferably water. A suitable precipitating agent, such as anammonium carbonate, ammonium bicarbonate or ammonium hydroxide is addedto the solution, thereby causing the metal to precipitate out as thecorresponding metal carbonate or hydroxide. The precipitate is thendried at a temperature greater than about 100° C., preferably from about105° C. to about 120° C., and more preferably at about 110° C. Afterdrying, the precipitate is mixed with a suitable dispersing agent andcalcined at a temperature from about 200° to 400° C., preferably fromabout 200° to about 300° C., thereby converting the individual metals totheir respective oxide form. The milled metal powder mixture can thenheated, in a hydrogen-containing atmosphere, at a temperature from about400° to about 600° C., preferably from about 450° to 550° C., for aneffective amount of time, to produce the catalyst in its metallic state.By effective amount of time, we mean that amount of time needed toreduce substantially all of the metal oxides to the respective metal oralloy having a suitable particle size. A typical amount of time willgenerally be from about 15 to 25 hours. Suitable particle sizes are fromabout 2.5 nm to about 150 nm, preferably from about 2.5 nm to about 100nm, and more preferably from about 2.5 nm to about 20 nm. Following thistreatment the chemically reduced catalyst is cooled to about roomtemperature in a helium environment before being passivated in a 2%oxygen/helium mixture for 1 hour at about room temperature (24° C.).

Salts of the catalytic metal used for growing the graphitic nanofibercatalysts of the present invention are salts that are soluble in water,organic solvents, and diluted mineral acids. Non-limiting examples ofwater-soluble salts suitable for use herein include nitrates, sulfatesand chlorides. Non-limiting examples of preferred salts soluble inorganic solvents, which are suitable for use herein, include formates,acetates, and oxalates. Non-limiting examples of organic solvents thatare suitable for use herein include alcohols, such as methanol, ethanol,propanol, and butanol; ketones, such as acetone; acetates and esters;and aromatics, such as benzene and toluene.

Carbon-containing compounds suitable for creating an atmosphere for thegrowth of the graphitic nanocatalysts of the present invention arecompounds composed mainly of carbon atoms and hydrogen atoms, althoughcarbon monoxide can also be used. The carbon-containing compound, whichis typically introduced into the heating zone in gaseous form, willgenerally have no more than 8 carbon atoms, preferably no more than 6carbon atoms, more preferably no more than 4 carbon atoms, and mostpreferably no more than 2 carbon atoms. Non-limiting examples of suchcompounds include CO, methane, ethane, ethylene, acetylene, propane,propylene, butane, butene, butadiene, pentane, pentene, cyclopentadiene,hexane, cyclohexane, benzene, and toluene. Combinations of gases arepreferred, particularly carbon monoxide and ethylene.

It may be desirable to have an effective amount of hydrogen present inthe heating, or growth, zone during nanostructure growth. Hydrogenserves two complementary functions. For example, on the one hand it actsas a reconstruction agent for the catalyst, suppresses the formation ofmetal carbide that results in deactivation and on the other hand ithydrogasifies, or causes carbon burn-off, of the carbon structure. By aneffective amount, we mean that minimum amount of hydrogen that willmaintain a clean catalyst surface (free of carbon residue), but not somuch that will cause excessive hydrogasification, or burn-off, of carbonfrom the nanostructures and/or substrate structure, if present.Generally, the amount of hydrogen present will range from about 5 to 40vol. %, preferably from about 10 to 30 vol. %, and more preferably fromabout 15 to 25 vol. %. For some catalyst systems, such as Cu:Fe, thehydrogasification reaction is relatively slow, thus, an effective amountof hydrogen is needed to clean the catalyst in order to keep it clean ofcarbon residue and maintain its activity. For other catalyst systems,such as Cu:Ni, where the activity is so high that excessivehydrogasification occurs, even at relatively low levels of hydrogen,little, if any, hydrogen is needed in the heating zone. A Cu:Ni catalystis so active that it utilizes essentially all of the carbon depositedthereon to grow nanofibers, and thus, there is generally no carbonresidue to clean off.

After the carbon nanofibers are grown, it is required to heat the finalstructure in an inert gas at temperatures up to about 3000° C.,preferably from about 1800° C. to about 3000° C., and more preferablyfrom about 2300° C. to about 3000° C. Under these conditions, thesurface area of the nanofibers are decreased because up to 50% of theadjacent edges of the nanofibers undergo a sealing action to form thetype of modified structure of the present invention.

As previously mentioned, the graphite nanofiber catalysts of the presentinvention are suitable for catalyzing a variety of chemical reactions.Non-limiting examples of chemical reactions that can be catalyzed withthe graphite nanofiber catalysts of the present invention includeoxidation, hydrogenation, oxidative-dehydrogenation, anddehydrogenation. One preferred oxidative dehydrogenation reaction is theconversion of ethylbenzene to styrene.

Below is a first table setting forth preferred hydrogenation reactionsalong with the typical catalytic metal used and reaction conditionsemployed.

Hydrogenation Reactions

Temperature Pressure Reaction Catalyst Range (° C.) (atm) Benzene tocyclohexane Ni 180-230 20-50 Nitrobenzene to Aniline Pd, Pt  50-150  1-5Reductive alkylation of Pt ˜50 ˜1 nitroaromatics Nitriles to amines Co,Ru, Ni  80-200 20-170 Hydrogenation of fats & oils Ni 120-175  1-2

Below is a first table setting forth preferred oxidation reactions alongwith the typical catalytic metal used and reaction conditions employed.

Oxidation Reactions

Temperature Pressure Reaction Catalyst Range (° C.) (atm) Sulfur dioxideto sulfuric acid V₂O₅/K₂O 420-480 ˜1 Ethylene to ethylene oxide Ag200-250 ˜8 Ethylene to vinyl acetate Pd  10-130 30 Propylene to acroleinBi₂O₃/Mo₂O₃ 320-430 2

It is evident that a number of factors can exert an impact on theultimate performance of the carbon nanofber catalysts. When dealing withactive carbons one generally considers the textural characteristics ofthe solid with particular emphasis being placed on the surface area andthe pore size distribution. Unfortunately, the small pore size of activecarbons appears to be responsible for obstructing desorption of thestyrene, which blocks surface sites and eventually poisons the catalyst.A further shortcoming of active carbons is their propensity to undergogasification at about 550° C., a temperature close to that where theoxidative dehydrogenation reaction is conducted. On the other hand,graphitic materials are more resistant to attack by oxygen. As aconsequence, such carbons are stable at 550° C. and would not besusceptible to poisoning by adsorption of styrene molecules.

The catalytic performance of the heat-treated graphite nanofibers isdependent upon the electrical conductivity of the materials. Thisproperty can be enhanced via intercalation with various electron donorand acceptor molecules. Inorganic molecules and compounds that can formintercalation compounds with the graphite nanofibers include Li, Na, K,Rb, Cs, Br₂, Cl₂, F₂, ICl, ICl₃, H₂SO₄, HNO₃, H₂SeO₄, HClO₄, H₃PO₄,H₄P₂O₇, H₃AsO₄, HF, CrO₂Cl₂, CrO₂F₂, UO₂Cl₂, FeCl, CuCl₂, BCl₃, AlCl₃,CoCl₃, RuCl₃, RhCl₃, PdCl₄, PtCl₄, Cr₂O₃, Sb₂O₃, MoO₃, Sb₂S₃, CuS, FeS₂Cr₂S₃, V₂S₃ and WS₂.

The present invention will be illustrated in more detail with referenceto the following examples, which should not be construed to be limitingin scope of the present invention.

EXAMPLES

Materials

The “platelet” graphitic nanofibers (P-GNF) used in these examples wereprepared from the decomposition of a carbon monoxide/hydrogen mixturesover a copper-iron powdered catalyst at 600° C. Prior to use, allnanofibers were treated in dilute mineral acid for a period of one weekto remove associated metal catalyst particles. Samples of thesenanofibers were subsequently treated in argon for 30 minutes at either1800° C. or 2330° C. Examination of these heat-treated materials byhigh-resolution transmission electron microscopy revealed that many ofthe adjacent edges had undergone a sealing action by generating loops atthe exposed regions.

The gases used in these examples were carbon monoxide (99.9%), ethylene(99.95%); hydrogen (99.999%), helium (99.99%) and argon (99.99%) werepurchased from Air Products and dried before use. Reagent grade ironnitrate, cobalt nitrate, nickel nitrate, copper nitrate and magnesiumoxide were used in the preparation of catalysts for carbon nanofibergrowth and were obtained from Fisher Scientific.

Example 1

The oxidative dehydrogenation of ethylbenzene to styrene was carried outin a packed bed tubular quartz flow reactor system. The flow rates ofthe gaseous reactants, oxygen and helium, were regulated by MKS massflow controllers. Ethylbenzene (EB) was introduced into the reactorusing a syringe pump. The inlet and outlet gas analyses were performedon-line using a gas chromatograph equipped with thermal conductivitydetectors (TCD) and flame ionization detectors (FID) detectors. Theperformance of each catalyst sample was determined from the conversionof EB, the selectivity to styrene (ST) and the resulting yield ofstyrene. These values were calculated according to the followingequations: $\begin{matrix}{{EB}_{conversion} = \frac{n_{{EB}_{i\quad n}} - n_{{EB}_{ex}}}{n_{{EB}_{i\quad n}}}} & (1) \\{{ST}_{selectivity} = \frac{n_{{ST}_{ex}}}{n_{{EB}_{i\quad n}} - n_{{EB}_{ex}}}} & (2) \\{{{ST}_{yield} = \frac{n_{{ST}_{ex}}}{n_{{EB}_{ex}}}}\quad} & (3)\end{matrix}$where, n is the number of moles of a given compound, “in” and “ex” referto inlet and exit, respectively.

In this series of experiments the behavior of high temperature treatedgraphite nanofibers for synthesis of styrene as a function of reactiontemperature was investigated. Table 1 below shows the behavior of themodified platelet graphite nanofibers (P-GNF) that was treated at 2330°C. in argon for 30 minutes. The reaction conditions were as follows:mole ratio O₂/EB=1.4, EB flow rate=9.33×10⁻⁶ mol/min, He=9.8 cc/min,catalyst weight=40.6 mg. In each case, the data were taken after 22hours on stream. Examination of these data shows that as the reactiontemperature is progressively raised there is a corresponding increase inthe EB conversion, reaching a maximum level at about 600° C. On theother hand, the optimum selectivity to the desired product, styrene, isachieved at 450° C. and the maximum yield of styrene occurs at 575° C.TABLE 1 EB ST Temperature (° C.) Conversion (%) Selectivity (%) ST Yield(%) 450 23.5 100.0 23.7 475 34.3 93.6 32.1 500 47.7 92.8 44.2 547 69.676.5 53.3 575 73.7 78.2 57.7 600 80.7 69.9 56.4 625 76.9 70.1 53.8 65076.4 61.6 47.1

Example 2

In this set of experiments the heat-treated P-GNFs described above werereacted in various ethylbenzene/oxygen mixtures at 500° C. Otherreaction conditions were as follows: EB flow rate=9.33×10⁻⁶ mol/min,He=9.8 cc/min, catalyst weight=40.6 mg and the data presented in Table 2were taken after 22 hours on stream. Inspection of these data shows thatas the O₂/EB ratio is increased there is a concomitant increase in theconversion of the hydrocarbon. However, as the O₂/EB ratio is raisedabove 1.0 the selectivity to styrene declines. The optimum conditionsfor the process appear to be an O₂/EB ratio of about 1.0. TABLE 2 O₂/ EBEB Mole Ratio Conversion (%) ST Selectivity (%) ST Yield (%) 0.5 25.899.9 25.8 0.86 39.1 100.0 40.4 1.0 43.8 99.7 43.6 1.4 47.7 92.8 44.2 1.948.8 91.2 44.1

Example 3

This series of experiments was conducted to establish the activitymaintenance and selectivity pattern of the modified P-GNF catalyst forthe ethylbenzene oxidative dehydrogenation reaction at 547° C. as afunction of time. Other reaction conditions were as follows: mole ratioO₂/EB=0.86, EB flow rate=9.33×10⁻⁶ mol/min, He=9.8 cc/min, catalystweight=40.6 mg. TABLE 3 Reaction Time (h) EB conversion (%) STselectivity (%) ST yield (%) 0.50 24.6 50.7 12.5 4.62 41.9 81.7 34.25.10 41.2 99.3 40.9 6.65 42.7 100.0 42.0 9.95 45.8 100.0 46.2 10.97 47.5100.0 48.3 12.48 51.0 94.7 48.3 15.80 51.2 93.9 48.1 18.30 53.8 93.250.1 22.73 53.9 89.8 48.3

Examination of the data presented in Table 3 shows that following aninduction period of about 5 hours, the catalytic activity of themodified P-GNFs for ethylbenzene conversion actually increases with timeand maintains a very high selectivity towards the formation of thedesired product, styrene for an extended period of time. This behavioris to be contrasted with that of the corresponding P-GNFs notheat-treated (Table 4), which undergoes deactivation after a relativelyshort time on stream while maintaining the selectivity for styreneproduction.

Example 4

A comparison study was carried out using P-GNFs not heat-treated andreacted under the same conditions as those used in Example 3. Thereaction conditions were as follows: reaction temperature 547° C., moleratio O₂/EB=0.86, EB flow rate=9.33×10⁻⁶ mol/min, He=9.8 cc/min,catalyst weight=40.4 mg. TABLE 4 Reaction Time (h) EB conversion (%) STselectivity (%) ST yield (%) 0.25 59.9 85.5 51.2 1.28 48.9 87.1 42.61.82 46.7 83.8 39.2 2.35 43.6 88.3 38.5 2.90 38.2 90.1 37.8 3.42 37.890.7 36.7 3.93 40.3 87.6 35.3 4.47 40.1 86.7 34.8 9.63 31.6 82.4 26.0

Inspection of the data given in Table 4 shows that the catalyticactivity declines over the reaction period while the selectivity patternremains unchanged. Clearly the performance of this system is inferior tothat displayed by the high-temperature treated P-GNFs.

Example 5

In this set of experiments the performance of the modified P-GNFscatalyst for the ethylbenzene oxidative dehydrogenation reaction at 500°C. as a function of time was investigated. TABLE 5 Reaction Time (h) EBconversion (%) ST selectivity (%) ST yield (%) 0.57 38.3 100.0 39.9 1.6844.5 95.3 42.4 2.73 42.0 100.0 43.1 3.22 43.5 100.0 43.6 3.72 45.2 94.642.8 4.27 43.7 99.4 43.5 4.73 44.4 100.0 45.2 5.25 41.3 100.0 43.5 6.0041.1 100.0 42.3 16.00 41.9 100.0 42.5

The catalyst had been previously utilized at higher temperature and wastherefore already in an activated state. Other reaction conditions wereas follows: mole ratio O₂/EB=0.86, EB flow rate=9.33×10⁻⁶ mol/min,He=9.8 cc/min, catalyst weight=40.6 mg. Examination of the data given inTable 5 reveals that over a period of 16 hours the catalyst performanceremains relatively stable. It is also apparent that under theseconditions that the high-temperature treated P-GNF catalyst exhibits anexceedingly high selectivity towards styrene production and moreover,this high level is maintain for the entire period of the reaction.

Example 6

In a further series of experiments the catalytic behavior of acommercial carbon black, XC72 was investigated. This material isavailable from Cabot Corporation and has a surface area of 230 m²/g andan average pore size of 5.2 nm. The conditions used were the same asthose used in Example 5. The oxidative dehydrogenation of ethylbenzenewas carried out at 500° C. for an extended period of time. Otherreaction conditions were as follows: mole ratio O₂/EB=0.86, EB flowrate=9.33×10⁻⁶ mol/min, He=9.8 cc/min, catalyst weight=40.5 mg. From theresults given in Table 6 it is evident that the catalyst exhibits aprogressive decrease in activity as the reaction proceeds. Furthermore,the selectivity towards styrene formation also declines with time onstream. A comparison of the performance of this type of carbon with thehigh-temperature treated P-GNFs shows that the latter material exhibitsa superior performance. TABLE 6 Reaction Time (h) EB conversion (%) STselectivity (%) ST yield (%) 0.40 41.6 90.3 37.6 1.48 35.6 98.9 35.23.52 35.1 94.2 33.1 5.23 33.1 98.8 32.7 9.82 31.8 92.1 29.3 12.53 35.580.3 28.5

Example 7

The data given in Table 7 shows the comparison of the performance ofvarious materials, including the current commercial system based onFe,Cr,K oxides, for the catalyzed oxidative dehydrogenation ofethylbenzene at 500° C. Other reaction conditions were as follows: moleratio O₂/EB=0.86, EB flow rate=9.33×10⁻⁶ mol/min, He=9.8 cc/min,catalyst weight=40.5 mg. The data were taken 17 hours after the start ofthe reaction. TABLE 7 Pore (%) EB (%) ST S.A. Size Catalyst conversionselectivity (%) ST yield (m²/g) (nm) P-GNF 2330° 39.1 100.0 40.4 40 13.2P-GNF 1800° 34.2 100.0 34.7 50 11.8 P-GNF 35.1 94.1 33.0 80 6.3 XC-7234.6 75.5 29.3 230 5.2 Fe, Cr, K oxides 6.9 75.9 5.2 4.4 4.0

Examination of the results shows some significant features andhighlights the superior performance of the P-GNFs that had been treatedin argon at 2330° C., which is significantly better than that of thesame type of GNF that had been heated to 1800° C. While both of thesematerials exhibited a 100% selectively towards styrene, it is thegeneration of a higher pore size in the former that appears to be thecritical factor. Indeed, when one considers all the data there appearsto be a direct correlation between pore size and catalytic performance.In sharp contrast, the magnitude of the surface area of the materialsdoes not have an impact on the catalytic behavior.

The electrical conductivity of high temperature treated “platelet”graphite nanofibers of the present invention were enhanced by theinteraction of concentrated nitric acid at 90° C. Under these conditionsHNO₃ species intercalates between the exposed graphite edge sites toinitially form a compound having the formula, C₆HNO₃ and after continueduptake, the intercalation compound, C₁₂HNO₃ is produced. The achievementof this final stage compound resulted in an enhancement of a factor of20 over that of the pristine material. Furthermore, these compounds arerelatively stable in air. The formation of an intercalation compound wasconfirmed by X-ray diffraction analysis in which the expansion of thed₀₀₂-spacing was measured.

1. A method for increasing the catalytic activity of graphiticnanofibers comprised of a plurality of graphite platelets that arealigned at an angle from about 1° to about 90° with respect to thelongitudinal axis of the nanofibers and which nanofibers have acrystallinity greater than about 90%, which method comprises heattreating said nanofibers in an inert gas environment at temperaturesfrom about 2300° C. to about 3000° C.
 2. The method of claim 1 whereinthe platelets are aligned substantially 90° with respect to thelongitudinal axis of the nanofiber.
 3. The method of claim 1 wherein theplatelets are aligned at angle from about 30° to about 60° with respectto the longitudinal axis of the nanofibers.
 4. The method of claim 1wherein the inert gas is selected from helium and argon.
 5. The methodof claim 1 wherein said graphitic nanofibers are intercalated with anintercalation component selected from the group consisting of Li, Na, K,Rb, Cs, Br₂, Cl₂, F₂, ICl, ICl₃, H₂SO₄, HNO₃, H₂SeO₄, HClO₄, H₃PO₄,H₄P₂O₇, H₃AsO₄, HF, CrO₂Cl₂, CrO₂F₂, UO₂Cl₂, FeCl, CuCl₂, BCl₃, AlCl₃,CoCl₃, RuCl₃, RhCl₃, PdCl₄, PtCl₄, Cr₂O₃, Sb₂O₃, MoO₃, Sb₂S₃, CuS, FeS₂Cr₂S₃, V₂S₃ and WS₂.
 6. A catalytic chemical reaction selected fromoxidation, hydrogenation, dehydrogenation, oxidative-hydrogenation, andoxidative-dehydrogenation which is catalyzed by a catalyst compositioncomprised of graphitic nanofibers which nanofibers are comprised of aplurality of graphite platelets aligned at an angle from about 1° toabout 90° with respect to the longitudinal axis of the nanofibers andwhich nanofibers have a crystallinity greater than about 90%, whichmethod comprises heat treating said nanofibers in an inert gasenvironment at temperatures from about 2300° C. to about 3000° C.
 7. Thecatalytic chemical reaction of claim 6 which is aoxidative-hydrogenation reaction.
 8. The catalytic chemical reaction ofclaim 7 which is the oxidative-hydrogenation reaction of ethylbenzene tostyrene.
 9. The catalytic chemical reaction of claim 6 wherein theplatelets are aligned substantially 90° with respect to the longitudinalaxis of the nanofiber.
 10. The catalytic chemical reaction of claim 6wherein the platelets are aligned at angle from about 30° to about 60°with respect to the longitudinal axis of the nanofibers.
 11. Thecatalytic chemical reaction of claim 6 wherein the inert gas is selectedfrom helium and argon.
 12. The catalytic chemical reaction of claim 8wherein the platelets are aligned substantially 90° with respect to thelongitudinal axis of the nanofiber.
 13. The catalytic chemical reactionof claim 8 wherein the platelets are aligned at angle from about 30° toabout 60° with respect to the longitudinal axis of the nanofibers. 14.The catalytic chemical reaction of claim 8 wherein said graphiticnanofibers are intercalated with an intercalation component selectedfrom the group consisting of Li, Na, K, Rb, Cs, Br₂, Cl₂, F₂, ICl, ICl₃,H₂SO₄, HNO₃, H₂SeO₄, HClO₄, H₃PO₄, H₄P₂O₇, H₃AsO₄, HF, CrO₂Cl₂, CrO₂F₂,UO₂Cl₂, FeCl, CuCl₂, BCl₃, AlCl₃, CoCl₃, RuCl₃, RhCl₃, PdCl₄, PtCl₄,Cr₂O₃, Sb₂O₃, MoO₃, Sb₂S₃, CuS, FeS₂ Cr₂S₃, V₂S₃ and WS₂.